HIGHLY SELECTIVE TRAPPING OF UNSYMMETRICAL PENTADIENYL ANIONS BY SILICON AND TIN ELECTROPHILES - MARKED CONTRAST BETWEEN KINETIC AND THERMODYNAMIC CONTROL OF PRODUCT COMPOSITION

The nucleophilic reaction of lithium and potassium salts of five pentadienyl anions bearing sterically and/or coordinatively different groups at their unsymmetrical 2-position toward Me3SiCl and Me3SnBr is described. When the anion of the potassium salt was trapped by Me3SiCl, (Z)-4-substituted-pentadienylsilanes were obtained with high selectivity (87-100%). The lithium salt, however, gave a mixture of (E)- and (Z)-4-substituted-pentadienylsilanes. The coupling reaction proceeds under kinetic control, because the product composition from Si trapping reflected the structure of the parent dienyl anion and the steric interaction between the incoming Me3Si group and the substituent on the anion. In contrast, when the anion was trapped by Me3SnBr under the identical conditions, (Z)-2-substituted-pentadienyltins were formed with 60-96% selectivity. Only relatively small changes in product composition, as a function of the cation or the substituent on the anion, were observed. These results could be explained by thermodynamic controlling factors, including a 1,5-Me3Sn shift, and a redistribution of pentadienyl groups under basic conditions.